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7

NBS SPECIAL PUBLICATION 400-18

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Semiconductor Measurement Technology:

The Destructive

Bond Pull Test
NATIONAL BUREAU OF STANDARDS

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I Bureau cf Standard*

>H- Semiconductor Measurement Technology:

, The Destructive Bond — Pull Test

e c

.

John Albers, Editor

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Jointly supported by:

The National Bureau of Standards,

The Defense Nuclear Agency, and

The Navy Strategic Systems Project Office

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary Designate

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued February 1976
Library of Congress Cataloging in Publication Data

Main entry under title:

Semiconductor measurement technology.

(National Bureau of Standards special publication; 400-18)

"Jointly supported by: the National Bureau of Standards, the Defense Nuclear Agency, and the Navy Strategic Systems Project

Office." Supt. of Docs, no.: C 13.10:400-18

1. Aluminum wire—Testing. I. Albers, John. II. Series: United

Slates. National Bureau of Standards. Special publication; 400-

18. QC100.U57 no.400-18. [TK3307] 602'.ls [621.8'54'021] 75- 619425

National Bureau of Standards Special Publication 400-18

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-18, 52 pages (Feb. 1976) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1976

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. CI 3.10:400-18). Price $1.25 (Add 25 percent additional for other than U.S. mailing).
TABLE OF CONTENTS

Semiconductor Measurement Teohnology

THE DESTRUCTIVE BOND PULL TEST

Page

Preface vi

1. Introduction 2

1.1 Units 3

2. Resolution-of-Forces Calculation 4

3. Fabrication of Test Specimens 9

4. Variables Involved in Resolution-of-Forces Calculation 11 4.1 Position of Hook ( e ) 11 4.1.1 Single-Level Bonds 11

4.1.2 Two-Level Bonds 11 4.2 Angle e 13

4.2.1 Introduction 13

4.2.2 Single-Level Bonds 13 4.2.3 Two-Level Bonds 13 4.3 Loop Height (h) 16 4.3.1 Single-Level Unannealed Bonds 16

4.3.2 Single-Level Annealed Bonds 18

4.3.3 Two-Level Bonds 20

5. Variables Not Involved in Resolution-of-Forces Calculation 22

5.1 Rate of Pull 22

5.2 Angle a 24

5.2.1 Introduction 24

5.2.2 Single-Level Bonds 24

5.2.3 Two-Level Bonds 25

6. Sensitivity Calculation ' 26

7. Summary 28

References 29

Appendix A. Resolution-of-Forces Calculation 31

Appendix B. Programs Used for Resolution-of-Forces

Calculation 33

Appendix C. Bond Pull Specifications for LargeDiameter Wire 40

Appendix D. Non-Destructive Wire Bond Pull Test 42

iii
LIST OF FIGURES

Page

1. Geometric variables for the bond pull test 6

2. Dependence of F t/F on e t for various ratios of

to 6

3. F ./F as a function of d/h for different values of H/h for

e w

* 1/4 7

4. F ,/F as a function of d/h for different values of H/h for

Ew2 1/4 . . . 7

5. Scanning electron micrograph of bond pad. array used for singlelevel studies 10

6. Pictorial illustration of two-level substrate 10

7. Measured pull strength of single-level bonds for different

hook positions 12

8. Measured pull strength of two-level bond pairs as a function

of the position of the pulling hook 12

9. Geometrical variables for the pull test for bonds made on twolevel substrates for pulling normal to the substrate 14

10. Measured pull strength of single-level bond pairs as a function of the angle of pull in the plane of the bond loop 14

11. Pull configuration for two-level bonds 15

12. Measured pull strength of bond pairs as a function of the

angle of pull in the plane of the bond loop 15

13. Measured pull strength as a function of the bond angle (e) 17

14. Pull strength of unannealed round-wire bonds as a function

of loop height 17

15. Pull strength of ribbon-wire as a function of loop height 19

16. Pull strength of unannealed round-wire bonds and annealed

round-wire bonds as a function of loop height 19

17. Measured pull strength of unannealed, round-wire, twolevel bonds as a function of loop height above the high pad 21

18. Normalized pull strength of unannealed, round-robin, two-level

bonds as a function of loop height above the high pad 21

19. Measured bond pull strength as a function of pull rate 23

20. Normalized pull strength as a function of pull angle, a, for

round-wire bonds with small and large deformation 23

21. Normalized pull strength as a function of angle of pull,

a, for two-level bond pairs 25

iv
Page

22. Normalized variation in calculated pull strength for the

terminal bond, AF(t), and die bond, AF(d), as a function

of pull angle 41 27

Al. Vector diagram for resolution-of-forces operative in bonded system during application of pulling force 32

CI. Minimum destructive bond pull breaking-force vs. wire

diameter for aluminum wire bonds in semiconductor devices 41

Dl. Stress-strain curves for annealed (A) and hard-drawn or stress-relieved (B) aluminum wire 43

LIST OF TABLES

Page

1. Typical results for resolution-of-forces calculation for

single-level and two-level bonds 8

2. Calculated dependence of AF(t) and AF(d) on pull test conditions for

a TO-18 transistor 27

Bl . HP-65 Program for Resolution-of-Forces Calculation 34

B2. Basic Program for Resolution-of-Forces Calculation 35

B3. Fortran Program for Resolution-of-Forces Calculation 38

CI . Bond pull limits 41

Dl. Summary of NDP force recommendations 43

V
PREFACE

The Semiconductor Technology Program serves to focus NBS efforts to enhance the

performance, interchangeability , and reliability of discrete semiconductor devices and

integrated circuits through improvements in measurement technology for use in specifying

materials and devices in national and international commerce and for use by industry in

controlling device fabrication processes. Its major thrusts are the development of

carefully evaluated and well documented test procedures and associated technology and

the dissemination of such information to the electronics community. Application of the

output by industry will contribute to higher yields, lower cost, and higher reliability

of semiconductor devices. The output provides a common basis for the purchase specifications of government agencies which will lead to greater economy in government procurement.

In addition, improved measurement technology will provide a basis for controlled improvements in fabrication processes and in essential device characteristics.

The work described in this report has been jointly sponsored by the Defense Nuclear

Agency, the U.S. Navy Strategic Systems Project Office (Code SP-23, administered by

Naval Ammunition Depot, Crane, Indiana), and the National Bureau of Standards.

vi
Semiconductor Measurement Technology:

THE DESTRUCTIVE BOND PULL TEST

John Albers, Editor

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

This report summarizes the work done at NBS on the destructive bond

pull test as applied to small-diameter (approximately 1 mil or 25 ym)

ultrasonically bonded aluminum wire. This work was performed during

the period from 1969 to 1974. The report begins with a v brief summary

of the calculation of the resolution-of-forces operative in the bond

system during the application of the pulling force. Next, comparisons

of the theoretical and experimental dependencies of the pull strength

on the variables involved in the resolution-of-forces calculation are

given. Some of the variables which are not directly involved in this

calculation are then considered and their effects on the measured

pull strength are presented. The report ends with a sensitivity calculation as to how well the variables must be controlled to maintain

the variability of the pull strength within given limits.

Bond pull specifications for large-diameter wire as well as recom- mended force levels to be used in the application of the nondestructive

bond pull test, both of which have resulted from the pull test work,

are considered in the appendices.

Key Words: Bond angle; bonding; bond pull test; bond-to-bond

spacing; large wire; loop height; microelectronics; non-destructive

bond pull test; position of hook, pull rate; pull strength; resolution-of-forces ; semiconductor devices; ultrasonic bonding; wire

bond.

1
1. INTRODUCTION

This report is an edited summary of the NBS effort on the destructive bond pull test. The rationale for this work lies in the widespread use of this test method in the electronics industry to evaluate the mechanical strength of wire bonds in semiconductor devices

and in the large gap between the use of the pull test and the acquisition of reproducible

calculable quantities which could be used to quantify the test results. Hence, at the

request of several agencies, and with the guidance of standards organizations, an extensive

survey of test methods for wire bonds in general [1] and an evaluation of the pull test in

particular were begun.

Initial in-house efforts to produce reproducible wire bonds to be used in the evaluation showed that the currently practiced industrial procedures were inadequate. This led

to a study of ultrasonic wire bonding techniques. The results of this study pointed to the

lack of shock and vibration isolation and the subsequent bond variability due to environmental and operator induced stresses as being primarily responsible for this inadequacy. An offshoot of this investigation was a better understanding of ultrasonic bonding through

the use of magnetic pickup, capacitor microphone, and low-power laser detectors of tool

motion during the bonding process. Optimized bond quality through bonding schedule studies resulted from this study. An exposition of a mechanism for the formation of ultrasonic

bonds also followed from this study. A detailed treatment of these facets of the NBS work

appears elsewhere [2],

Once bonder and related problems were solved, work was begun to quantify the results

of the pull test. In order to carry out this program, it was necessary to correlate the

measured pull strength as determined in the pull test with the stress in the wire, which

depends on the geometry of the bond system. This was done through the resolution-of-forces calculation. An extensive series of experiments was undertaken in order to relate observed

pull strengths to the results of the resolution-of-forces calculation. To further enhance

the understanding of the pull test, certain variables which could not be introduced in the

two-dimensional resolution-of-forces calculation were experimentally studied to ascertain

their effects on the pull test results. This report is concerned with the results and

conclusions of this investigation.

The results of the pull test work have been used by ASTM Committee F-l on Electronics

in the preparation of a document on the pull test method. At the time of this writing,

the document is well on its way to becoming an accepted standard. To support this evolution, a round robin is presently being run using test vehicles prepared at NBS. On each

specimen, three groups of bonds of different deformation were prepared. One-half of the

bonds of each group were pulled to destruction at NBS and the remaining half were pulled

at the individual laboratories participating in the round robin. The round robin is not

complete; however, preliminary results indicate that the mean pull strengths as determined

by NBS and by each of the other participating laboratories differ by about ten percent. It should be noted that these conclusions are preliminary and are based upon raw data. A more

2
complete treatment of the round robin and the data analysis will appear in the future when

the round robin is complete.

The better understanding of the pull test has led to other developments. These include

a bond pull specification for large diameter wire as well as statistical and metallurgical

rationales for a nondestructive wire bond pull test. These are discussed in appendices C and D respectively. The work on the nondestructive pull test is also being used by ASTM

Committee F-l on Electronics in the development of a recommended practice for carrying out

this type of test. The following section contains a brief summary of a calculation of the resolution of

the forces operative in the bond system during the application of the pulling force. A

more complete derivation is presented in Appendix A and programs (in several different

languages) which may be used for numerical calculation of the resolution-of-forces equations

are given in Appendix B. The discussion of the resolution-of-forces calculation is followed in Section 3 by a description of the fabrication of the specimens used for the single-level and two-level bond

studies

.

In Section 4, comparisons of the theoretical and experimental dependencies of the pull

strength on the variables which are involved in the resolution-of-forces calculation are

given. Of particular importance are the loop height, the position of the hook, and the

angle between the direction of the pulling force and the normal to the line joining the

bond terminals. In Section 5, consideration is given to some of the variables which are not directly

involved in the resolution of forces calculation. The variables considered are the rate

of pull and the angle between the direction of pull and the normal to the substrate in the

plane perpendicular to both the substrate and the plane of the bond loop.

Section 6 contains a sensitivity computation from which it can be estimated how well

the variables must be controlled in order to maintain the accuracy of the results to within

a given interval. A summary of the results comprises the final portion of the report.

1.1 UNITS

The American semiconductor industry has traditionally used mixed English and metric

units, but presently there is a trend in the direction of the International System (SI)

units. For the purposes of conversion, it should be noted that 1 mil = 0.001 in. = 25.4 um

and that 1 gram force (gf) =9.8 millinewtons (mN)

.

3
2. RESOLUTION-OF-FORCES CALCULATION

The destructive bond pull test, more commonly called simply the pull test, is one of

the most widely used test methods employed in the evaluation of the quality of wire bond

systems. In its elemental form, the test consists of pulling the wire span between the

bond on the terminal and the bond on the semiconductor die with a hook assembly until

rupture takes place. The pulling force at which rupture occurs is usually referred to as the pull strength of the bond. The pull force at rupture is most frequently presented in

units of grams force.

An analysis of the resolution-of-forces provides relationships between the pull

strength and the forces in the bond system at rupture. In figure 1, the geometrical

variables are presented for a typical two-level bond. The angle <f> is that between the

direction of the applied force F and the normal to the substrate. At the bond on the

semiconductor die, the angle between the wire and the surface of the die is denoted by 9^. At the terminal, the angle between the wire and the plane of the terminal surface is dewt noted by 9 . The forces in the wire at the terminal and die are designated by Fit1_ and F respectively. These forces are related to the applied pull force as follows:

(1)

wd'

F = F wt

F = F wd

cos (9 ,-<f>) Q

sin(9 +9 ,) t d _ cos(9

t

+(j>)

sin (9 +9 ,) t a

(2)

For the simple case of a single-level bond pulled normal to the substrate and at the

middle of the wire span, §=0 and 9 t=9^=9. In this instance, the above equations reduce to

F = F = F — wd wt 2sin9 (3)

The angular variables 9 and 9, are often difficult to measure. However, the angles

t d may be related to the variables h, H, d, and e which are more easily accessible to measurement. It may be seen from figure 1 that h is the height of the wire span (at the point of

pull) above the terminal contact surface. The vertical distance between the terminal contact surface and the semiconductor die surface is denoted by H. The horizontal distance

between the bond on the terminal and the bond on the die is denoted by d and is usually

referred to as the bond-to-bond spacing. Finally, e, where OseSl, is the dimensionless

fractional horizontal distance between the terminal bond and the point of application of

the pulling force. In terms of these distance variables, F „ and F are related to F for

wd wt

a two-level bond as follows

:

F = F wt till(l-e)cos<j>+ (h+H)sin*j

(4)

h
If the bond is pulled normal to the substrate ($=0) , eqs (4) and (5) reduce to

For a single-level bond which is pulled at mid-span, in a direction normal to the

substrate, both <|> and H are zero and e is equal to one-half. Under these conditions,

eqs (4) and (5) reduce to

F =F = 4l + (^)T2

* (8)

wt wd 2|_ v 2h' J

It should be noted that eqs (1) through (8) may be used for calculating pull strengths

only when heel breakage (tensile failure) is observed.

For the convenience of readers interested in numerical computations, Appendix B contains programs in several different languages which make use of eqs (4) and (5) to calculate the pull strength. For purposes of graphical illustration of the predictions

of eqs (1) through (8) , plots of the force in the wire on the terminal or die divided by

the pulling force as functions of the angular variables or the length variables are presented in figures 2 to 4. Also, table 1 contains typical results from the resolution-offorces calculation for single-level and two-level bonds [3].

5
Figure 1. Geometric variables for the bond pull test.

Figure 2. Dependence of F . /F on e t for various ratios 0^ to e t (The curves are for the case $ = 0. Horizontal bars above and

below the curves for 26^ = e t show the effect of changing <f> to plus and minus 5 deg, respectively. To obtain the dependence of

Fwt/F, interchange everywhere the subscripts d and t and change

the sign of <j>)

.

6
d/h

Figure 4. F

W(j/F as a function of d/h for different values of

H/h and for e = 1/4.

7
Table 1. Typical Results from Resolution-of-Forces Calculation for

Single-Level and Two-Level Bonds

Single Level H = 0 d = 1 . 0 mm F = 10 gf

h(mm) e <j>(deg) Fwt(gf) Fwd(gf)

0.1 0.50 0 25.5 25.5

0.2 0.50 0 13.46 13.46

0.3 0.50 0 9.72 9.72

0.4 0.50 0 8.00 8.00

0.5 0.50 0 7.07 7.07

0.3 0.25 0 9.76 6.73

0.3 0.50 0 9.72 9.72

0.3 0.75 0 6.73 9.76

0.3 0.50 -10 8.56 10.58

0.3 0.50 0 9.72 9.72

0.3 0.50 10 10.58 8.56

Two Level H = 0.5 mm d = 1 . 0 mm F = 10 gf

h(mm) e 4>(deg) Fwt(gf) Fwd(gf)

0.1 0.50 0 7.28 11.16

0.2 0.50 0 5.98 9.56

0.3 0.50 0 5.30 8.58

0.4 0.50 0 4.93 7.92

0.5 0.50 0 4.71 7.45

0.3 0.25 0 6.89 6.45

0.3 0.50 0 5.30 8.58

0.3 0.75 0 2.99 9.31 0.3 0.50 -10 3.75 9.34

0.3 0.50 0 5.30 8.58

0.3 0.50 -10 6.69 7.55

8
3. FABRICATION OF TEST SPECIMENS

The bonding pads which were used for the work reported were fabricated on 0.010-in.

(0.25-mm) thick, 1.0-in. (25-mm) diameter silicon wafers, coated with 0.5 pm of steamgrown silicon dioxide and 0.5 to 1 um of evaporated aluminum. The aluminum was etched to leave 0.005-in. (0.13-mm) square bonding pads by means of standard photolithographic

techniques. An example of the bonding pad array is given in figure 5 [4]. For two-level studies, strips of silicon 0.05-in. (1.3-mm) wide, obtained by scribing

and breaking a prepared substrate, were affixed with a high temperature glue to another

substrate so that the bonding pad arrays lined up as shown in figure 6. Thus two-level

substrates were obtained with rows of bonding pads raised by the thickness of the silicon

strip (about 0.25 mm) above the bonding pads on the base substrate. It was necessary to position the silicon strip carefully on the base substrate so that a constant spacing between the upper pad and the lower pad was maintained along the length of the strip [5]

.

Except where otherwise indicated, the wire bonds tested were 0.001-in. (25-ym) diameter aluminum (1% silicon) wire ultrasonically bonded to the aluminum pads. Also, except

where otherwise noted, the bonds were made on a single bonding machine and were tested so

as to establish their reproducibility. The variables which were constrained by the mode of

fabrication were d, the bond-to-bond spacing, and H, the height of the terminal above the

die. For the experiments reported, d has the value of 1.0 mm and H has the values of zero (for single-level bonds) or 0.25 mm (for two-level bonds).

The bonding tools used to fabricate the bonds had a smooth front and a sharp back.

The sharp back assists the clamp in severing the wire after the second bond has been formed.

However, the sharp back of the tool causes the first bond heel to be slightly more deformed

than the second bond heel. The result is that the second bond is stronger under tensile

failure (heel breakage) for the same force-power-time combination than the first bond.

As the resolution-of-forces applies to the heel breakage failure mode, the forcepower-time combination for bond fabrication was chosen, except as otherwise indicated,

such that heel breakage was the principal failure mechanism.

9
Figure 6. Pictorial illustration of two-level substrate.

10
4. VARIABLES INVOLVED IN THE RESOLUTION

OF FORCES CALCULATION

In this section, consideration is given to the variables which are involved in the

resolution-of-forces calculation. These are the position of the hook, e, the angle 3 (the

angle formed between the direction of the pulling force and the normal to the line joining

the bond terminals), and the loop height, h. In studying each variable, an extensive

series of experiments was carried out in order to compare the prediction of the resolution

of forces with the measured pull strengths.

4.1 POSITION OF HOOK (e)

4.1.1 SINGLE-LEVEL BONDS

An experiment was performed to demonstrate the effect of the position of the pulling

hook on the measured pull strength of a single-level ultrasonic bond pair. The results of

the experiment are shown in figure 7 in which the measured pull strength, F, of the bond

pair is plotted against the position of the hook along the span of wire. The plotted

points are the mean pull strength and the error bars represent the 95 percent confidence

intervals for the mean. These experiments indicate that when the wire is pulled on the side nearest the first

bond, it is this bond that breaks at the heel due to tensile failure. Conversely, the

second bond breaks by tensile failure when the force is applied nearest to it. A transition point, where the probability for either bond to break is nearly the same, occurs at the maximum value of measured pull strength. In the present case this point is found to be

displaced more toward the second bond, as might be expected since the bonding machine used

in these tests yields a stronger second bond than first.

It should be noted that limited tests on aluminum wire doped with 1% magnesium rather

than silicon indicated, as expected, that there is no difference with respect to the dependence of the pull strength on the geometrical variables. The resolution of forces calculation relates the forces to geometrical variables only, while wire metallurgy determines

the tensile strength of the wire [6].

4.1.2 TWO-LEVEL BONDS

The effect of varying the position of the pulling hook on pull strength was studied

on two-level bond pairs where the first bond was made either to the low pad or to the high

pad. Specimen preparation procedures restricted the study to a single value of H, the

height of the terminal above the die. The results, plotted in figure 8 as measured pull

strength normalized to its value at midpoint as a function of hook position, are in

agreement with calculations based upon tensile failure except for positions between midpoint and the low pad. It should be noted that in this region the pulling hook was observed to slip along the wire loop toward the midpoint position; thus the measured pull

strength lies above the predicted value [5].

11
3 90 I

I-co| 8.0

I

7.0

a:

<2 6.0 <

1 I

I

1st BOND BREAKS

1 1 2nd BOND BREAKS

J

0.5 075

POSITION OF HOOK (mm)

Figure 7. Measured pull strength of single-level bond pairs for

different hook positions. (The first bond is at 0 and the second

bond is at 1 mm).

0.2 0.4 0.6 0.8

POSITION OF HOOK (mm)

1.0

Figure 8. Measured pull strength of two-level bond pairs as a function of the position of the pulling hook. (The values are normalized to the mean pull strength at mid span, 0.5 mm. Solid

points are for the first bond made to the high pad; open points

are for the first bond made to the low pad. In each case the

second bond is at 0 mm and the first bond is at 1.0 mm. The

data points represent the mean of 10 bonds all of which ruptured

by tensile failure at the heel of the first bond. Error bars

indicate one sample standard deviation above and below the mean. The solid curves were calculated by resol ution-of -forces.

)

12
4.2 ANGLE 3

4.2.1 INTRODUCTION

While the discussion of the resolution-of -forces has been cast in terms of the pull

angle <\> it is often more convenient to represent the results of pull test experiments

(particularly for two-level bonds) in terms of a different angle. This angle, denoted by

3, is defined as that formed between the direction of the pulling force and the normal

to the line joining the bond terminals (positive in the direction toward the second bond)

.

The angle 3 is related to the angle <f> by the equation 3 = 3 ±<t> where the positive sign

obtains if the first bond is the higher (in which case 3 Q

, the angle between the substrate

and the line joining the bond terminals, is negative), and the negative sign obtains if the

first bond is the lower (in which case 3 Q is positive). The various angles are illustrated

in figure 9.

4.2.2 SINGLE-LEVEL BONDS

Single-level bond loops were pulled at the midpoint of the loop at angles between 0 and 45 deg. in both directions. It should be noted that 3 Q

= 0 for single-level systems

and hence 3 = <}>. The results are shown in figure 10. The mean values of the measured

pull strength are denoted by the plotted points and the error bars represent the 95 percent

confidence intervals for the mean. The results show higher pull strengths for 3 negative

which is to be expected since the second bond is stronger than the first. When the larger

component of the force is applied to the first bond, the measured pull strength is lower

and when the larger component of the force is applied to the second bond, the measured

pull strength is higher [6].

4.2.3 TWO-LEVEL BONDS

The results of the single-level experiments for the larger values of 3 yielded some

insight into the pull test for two-level bond pairs. For a two-level bond system, there

are two ways the bond loop may typically be positioned for pulling as shown in figure 11. First, the substrate or package may be held level, which places each of the bonds at different heights, and the force applied in a direction normal to the substrate. Alternatively, the substrate may be tipped to bring both bonds to the same vertical height; the con- figuration produced is more nearly similar to a single-level bond case, and the force is

applied in a direction normal to an imaginary line joining the two bonds. The configuration in the level substrate case is similar to that of a single-level bond system where

the pulling is done at some angle 3 Q

. It is apparent that tipping or not in the two-level

bond system is essentially equivalent to varying the angle 3. Although this comparison

of single-level and two-level bond systems is in terms of bond loop geometry only, one would expect that the general results shown in figure 10 would be applicable to the twolevel system. That is, if the angle of pull is inclined toward the second bond (applying

the larger component of force to the first bond) the measured pull strength is less than

if the angle of pull is inclined toward the first bond (applying the larger component of
Figure 9. Geometric variables for the pull test for bonds made

on two-level substrates for pulling normal to the substrate.

(For this case, the angle cf> between the normal to the substrate

and the pulling direction is zero. The distance d is the total

bond-to-bond spacing in the plane of the substrate.)

,10

|

1 I

1 I

1 l

<—\—<—i—r-|—i—i i

|

i

^ q

[ I I i I i 1 i 1 I i I i I i I -40 -30 -20 -10 0 10 20 30 40 50 ANGLE OF PULL $ (deg)

Figure 10. Measured pull strength of single-level bond pairs as

a function of the angle of pull in the plane of the bond loop.

(The inset shows the relationship of the angle of pull to the

normal to the line joining the bonds. Note that the angle is

positive when the pull force is inclined toward the second bond

and negative when the pull force is inclined toward the first

bond.

)

14
t

a. Level substrate; nominal pull angle 3 = 3

b. Tipped substrate; nominal pull angle 3=0.

Figure 11. Pull configurations for two-level bonds. (The

direction of pull is indicated by the arrow.)

1.4 i r

1.3o

O

2 1 2

0E

-I 1 .1 —

I

r>

CL 1.0

0.9 o

0.8

A O

o

J L

-20 -10 0 10 20 30 40

ANGLE OF PULL 0 (deg)

Figure 12. Measured pull strength of bond pairs as a function

of the angle of pull in the plane of the bond loop. (The

values are normalized to the mean pull strength at 0 deg.

Solid points are for two-level bond pairs made with the first

bond to the high pad (circles) and with the first bond made

to the low pad (triangles); open circles are previously re- ported data for single-level bond pairs. Except for the solid

circles, which represent the mean of 30 bonds, the data points

represent the mean of 10 bonds, all of which ruptured at the

heel of the first bond. Error bars have been omitted to re- duce clutter; typically the sample standard deviation is be- tween 0.05 and 0.1 .

)

15
force to the second bond) . However, it should be noted that this analogy does not take

into consideration the effects of the angles of the wire at the bond heels for the two situations

.

To confirm this hypothesis, the effect of varying the pull angle 3 was studied for two- level ultrasonic bond pairs. Measurements of pull strength were made by tilting the sub- strate while pulling the loop vertically. The results are shown in figure 12 both for bond

pairs with the first bond made to the high pad and for bond pairs with the first bond made

to the low pad. The pull angle shown is 8 for the former case and -6 for the latter. For comparison, data for single-level bond pairs are also included; in this case the pull angle

is <j>, the angle between the direction of pulling force and the normal to the substrate. It can be seen that the trend in all three cases is about the same [5].

4.3 LOOP HEIGHT (h)

4.3.1 SINGLE-LEVEL UNANNEALED BONDS

A series of tests was conducted to determine the effect of variations in bond angle

(or ratio of loop height to bond-to-bond spacing) on the measured pull strength of singlelevel bond pairs pulled at the center of the loop in a direction perpendicular to the plane

of the pads. Eight groups of 10 single-level bond pairs were made with different loop

heights and spacings. The bond angle varied from 25.7 to 50.3 deg. The bond pairs were

then pulled to destruction. The exclusive failure mode was breakage at the heel of the

first bond.

Results of the experiment are shown in figure 13. The mean measured pull strength and

95 percent confidence intervals for the mean are plotted for each group against the bond

angle. The solid curve was calculated from the resolution-of-forces equation for the

symmetrical case under study:

F = 2F sin 6, (9) w

where F is the measured pull strength, F is the force in the wire, and 0 is the bond angle w

[7]. A further study was undertaken of the dependence of measured pull strength on loop

height or bond angle to gain greater insight into the extent of applicability of the reso- lution-of-forces calculation. Analysis of the results, shown in figure 14, supports the

conclusion that the pull strength calculated by resolution-of-forces does fit the experimental data. Above h/d = 1/3, several of the data points fall below the predicted value

of pull strength. This may be explained as follows. As the loop height increases, more

normal pulling force is exerted on the bonds. The results of the study of the influence of

the bond angle a (see sec 5.2) suggest that this might cause some peeling of the bond before

breakage at the heel occurs, which in turn, by stressing a more deformed region, might lower

the measured pull strength [8].

This conclusion is reinforced by the results of measurements of pull strengths of

16
35 40 45 6 (degrees)

Figure 13. Measured pull strength as a function of bond angle

(e), (The solid line, F = 17.2 sin e, is calculated from the

resolution-of-forces; the numerical factor is twice the breaking force in the wire. The error bars represent the 95 per- cent confidence intervals for the means.)

6 8 10 12 14»ia3 LOOP HEIGHT (in )

Figure 14. Pull strength of unannealed round-wire bonds as a function of loop height. (The data points represent the mean

for up to 10 bonds; only bonds which ruptured at the heel were included. Error bars represent one sample standard deviation. The solid curve is calculated from resolution-offorces using the wire tensile strength indicated by the arrow.)

17
0.0015 in x 0.0005 in (38 pm x 13 ym) ribbon-wire bonds as a function of loop height shown

in figure 15. The solid data points are average pull strengths for groups of up to 10

bonds each, excluding those bonds that failed due to lift-off of one of the bond pairs. For the two open data points at the two highest loop heights, values resulting from bond

lift-offs were included in the average pull strength calculation. Up to a loop height of

about 0.010 in. (0.25mm), the variation of pull strength as a function of loop height is in

agreement with the resolution-of -forces calculation, but at larger loop heights, the measured pull strength is lower than the predicted value [9].

4.3.2 SINGLE-LEVEL ANNEALED BONDS

A series of experiments was performed to determine the pull strength variation with

loop height of annealed single-level bonds. On a single substrate, groups of bond pairs

were made at four loop heights with a nominal bond-to-bond spacing of 0.040 in. (1.0 mm).

About half of these bonds were pulled to breaking in groups of ten at five different rates

of pull. The remaining bonds were annealed at 505°C for 25 min. and then groups of ten were pulled at each of the five rates. In agreement with previous measurements, the unan- nealed bonds did not exhibit any dependence of pull strength on rate of pull. Although

there is an indication that the annealed bonds show some dependence of pull strength on rate of pull, the major problem encountered in the interpretation of the measurements on annealed bonds lies in the fact that there is considerable elongation of these bonds during

pulling. The loop heights of annealed bonds at bond rupture are substantially larger than

they are in the original unannealed state. In figure 16, pull strength is shown as a function of loop height at rupture for the unannealed and annealed bonds studies. The differences in loop height between the annealed bonds and the unannealed state at which all bonds were made is evident by comparing the loop height of each of the four unannealed points and

their respective annealed points. As before, experimental data for unannealed bonds is in

good agreement with that predicted by the resolution-of-forces analysis (solid curve) up to

a loop height that corresponds to an h/d ratio of about 1/4. No such agreement could be obtained for the data (solid points) obtained from annealed bonds. However, it can be seen

that most of these points are for loop heights much larger than those for which agreement

between experiment and theory is obtained for unannealed bonds. This lack of agreement has

been attributed to the weakening effect of bond peel at large loop heights.

From the unannealed bond data, the percentage increase in pull strength required to bring the grand average of all pull strength group means at each loop height to coincidence

with the theoretical curve was calculated. A linear relationship was obtained between percent correction in pull strength against measured pull strength. This was extrapolated to the larger loop height values appropriate to the annealed bonds. When the annealed bond

data were corrected using this curve, the open points shown in figure 16 were obtained. It can be seen that these points appear to be in reasonable agreement with the theoretical

curve shown [10].

18
16" 10"3 LOOP HEIGHT (in.) Figure 15. Pull strength of ribbon-wire bonds as a function of

loop height. (The data points represent the mean for up to 10

bonds. Solid circles include only bonds which ruptured at the

heel; open circles include bonds which ruptured at the heel and

which failed by lift off. Error bars represent one sample

standard deviation. The solid curve is calculated from resolution-of-forces using the wire tensile strength indicated by the arrow.

)

10 1—

r

9 _ 8

en I 7

I-O

| 6

If) _i 5

_iz>

°- 4

3 ANNEALED

0 0.1 0.2 0.3 0.4 0.5 LOOP HEIGHT (mm)

Figure 16. Pull strength of unannealed round-wire bonds and

annealed round-wire bonds as a function of loop height. (Each

solid data point represents the mean for a group of 10 bonds,

the error bar represents one sample standard deviation. See text for a discussion of the open data points. The solid curves are calculated from resolution-of-forces using the wire

tensile strength indicated by the arrows: U - unannealed,

A - annealed).

19
4.3.3 TWO-LEVEL BONDS

For these loop height experiments, the two-level bonds had the following values of the

variables held constant: d = 1.0 mm, £=0.5, <}> = 0, H = 0.25 mm. Groups of 10 bonds were

made at 7 different loop heights. This was done both for the first bond made on the high

pad and for the first bond made on the low pad. In all cases failure occurred at the heel

of the first bond. The measured pull strengths are plotted against loop height in figure

17 for both cases (solid points - first bond high, open points - first bond low). The

plotted points represent the mean value of the 10 bond pulls and the error bars indicate

one sample standard deviation on either side of the mean. The solid lines represent the

values predicted by a resolution of the forces for each case. The agreement between the

trend of the theoretical curve and that of the experimental points for the first bond made

to the high pad appears to be good for loop heights less than about 0.3 mm. For higher

loops, the apparent pull strength is reduced as observed for both single level round and

ribbon wire bonds (see sec 4.3.1).

The results for the case of the first bond made on the low pad generally show overall

pull strengths less for a given loop height than those for the first bond made on the high

pad. This is in agreement with the experimental measurement of the effects of angle g (see

fig 12) in which higher pull strengths were observed for negative values of the pull angle

3 than for positive values [11].

This experiment was also carried out on bonds prepared on a different bonding machine.

The results of this experiment are presented in figure 18 where the normalized pull strength

is plotted against loop height. The results are in agreement with the above observations

[5].

20
12

2 I

1

I i I i

I 0 0.1 0.2 0.3 0.4

LOOP HEIGHT (mm)

Figure 17. Measured pull strength of unannealed, round-wire,

two-level bonds as a function of loop height above the high

pad. (Solid points are for the first bond made on the high

pad; open points are for the first bond made on the low pad.

The data points represent the mean of 10 bonds, all of which

ruptured at the heel of the first bond. Error bars indicate

one sample standard deviation above and below the mean. The

solid curves were calculated by resolution-of-forces.

)

0 0.1 0.2 0.3 0.4 0.5 LOOP HEIGHT (mm)

Figure 18. Normalized pull strength of unannealed, roundwire, two-level bonds as a function of loop height above the

high pad. (The values are normalized to the mean pull strength

at the lowest loop height. Solid points are for the first bond

made on the high pad; open points are for the first bond made

on the low pad. The data points represent the mean of 10 bonds,

all of which ruptured at the heel of the first bond. Error

bars indicate one sample standard deviation above and below

the mean. The solid curves were calculated by resolution-offorces. The bonds used in this experiment were made on a different bonding machine from that used to provide the data

for figure 17.)

21
5. VARIABLES NOT INVOLVED IN RESOLUTION-OF-FORCES CALCULATION

In this section, two of the variables which are not involved in the resolution-offorces calculation are considered. These variables are the rate of pull and the angle a (the angle between the direction of pull and the normal to the substrate in the plane

perpendicular to both the substrate and the plane of the undisturbed bond loop).

5.1 RATE OF PULL

As the resolution-of-forces calculation is of a static nature, an investigation was undertaken to see if there were appreciable effects on the pull strength brought about by

any transient effects induced by a rate dependent pull force. This is especially important

as in industrial use of the pull test the wire is frequently pulled rapidly or jerked which

may give rise to large transient effects. This is of potentially greater importance as the

elongation of the wire increases, since the effects of larger transients may be accentuated

by greater wire elongation.

In a first series of experiments, a group of 10 unannealed bonds was pulled to destruction at 11 different pull rates ranging from 1.0 gf/s (9.8 mN/s) to 12.5 gf/s (122 mN/s)

.

The results of this experiment are presented in figure 19 where the mean pull strength (with

the 95 percent confidence intervals for the mean) is plotted against the pull rate. The

slope of the curve was determined to be essentially zero. It was, therefore, concluded that

the pull rate has no significant effect on the bond pull strength for the range of pull

rate between 1.0 and 12.5 gf/s (9.8 to 122 mN/s) [12]. In a second series of experiments, the investigation of the effect of the rate of pull

on pull strength was extended to cover rates of 1.0 and 77 gf/s (9.8 and 764 mN/s). As with the first series of experiments, all bonds were made with the same bonding schedule in

sequence by one operator. Also, this schedule was set up such that the failure mode was breakage at the heel of the first bond. Groups of 60 bonds each were then pulled at rates

of 1.0 and 77 gf/s (9.8 and 764 mN/s) resulting in times to pull a bond of approximately 9 and 0.12 s. The date obtained indicated that there was no statistically significant dift ference in the measured pull strengths for either pull rate from those in figure 19. The above experiments were performed on single-level unannealed bonds where heel

breakage was the failure mode. For bonds which fail by peeling or lift-off, it is assumed

that the rate of pull would have a large effect on the measured pull strength since peel

failure appears to be rate sensitive. However, it is very difficult to show this statistically as a procedure to make bonds reproducibly that will fail by peeling has not been

found. Attempts have resulted in pull strengths which have greater variations from bondto-bond, at a given pull rate, than between groups pulled at different rates. Therefore,

a pull-rate experiment as described was not performed for bonds which fail by peel. However, it appears that the nature of the failure mode may affect comparisons of pull strengths.

Recent work (Kessler, H. K . , in Semiconductor Measurement Technology: Progress Report,

Bullis, W. M., Ed., NBS Special Publication 400-25 - to appear) indicates that the pull

strength of gold wire is also pull-rate independent.

22
PULL RATE (groms/second)

Figure 19. Measured bond pull strength as a function of

pull rate. (The error bars represent the 95 percent con- fidence intervals for the mean.)

i 1.1 H

0

Z

UJa 1.0 »-

~i 1 r -i 1 r

0.9 a

HiN 0 8 <

i

0.7

} i

1 I

5

1 I

0 10 20

ANGLE OF PULL a (deg

)

Figure 20. Normalized pull strength as a function of pull

angle, a, for round-wire bonds with small (•) and large (a)

deformation. (Error bars represent the 95 percent confidence

interval for the mean. The values are normalized to the mean

pull strength at each deformation at a = 0 deg.)

23
and rates of pull [13].

5.2 ANGLE a

5.2.1 INTRODUCTION

The resolution-of-forces calculation as presented in Section 2 and the Appendix of this

report is essentially a two-dimensional calculation. That is, only those forces operative

in the plane of the bond loop are considered. A series of experiments was undertaken to investigate the effects of pulling the bond in a direction not in this plane on the measured

pull strength. The angle a is defined as that between the direction of pull and the normal

to the substrate in the plane perpendicular to both the substrate and the plane of the un- disturbed bond loop. Since the principal effect of pulling at a finite value of a is to rotate the plane of the bond loop, the ratio of the force of pull and the force exerted

along the wire is the same for all values of a. This would lead one to conclude that no great dependence on the measured pull strength on a would be expected. However, the rotation of the plane of the bond loop does not include rotation of the bond heel. This tends

to put the bond heel under an anisotropic stress with respect to the line between the bonds.

This anisotropic stress may be expected to be accentuated by greater bond deformation. This

reasoning points to the possibility of decreased measured bond pull strength with increased

a- This decrease is expected to be most pronounced with a large deformation.

5.2.2 SINGLE-LEVEL BONDS

Single-level round-wire bonds were made on three metallized substrates with 0.001-in.

(25-um) diameter aluminum (1% silicon) wire and a bond-to-bond spacing of 0.04 in. (1.0 mm).

All the bonds on a particular substrate were made with the same power setting, but different

power settings were used in making the bonds on different substrates to obtain bonds with

different degrees of bond deformation. On each substrate, about 35 bonds were pulled at the

midpoint of the loop for eight values of a from 0 to 20 deg. The pull angle <t> , in the plane

of the bond loop, was 0 deg. For each group, the mean pull strength and the 95 percent con- fidence interval for the mean was determined; the measured pull strengths of bonds that did not break at the heel were excluded from the calculations. The pull strengths were then

normalized to the mean value obtained at a = 0 deg. The results for bonds on two substrates

with small deformation (1.25 to 1.50 wire diameters) and large deformation (2.25 to 2.50

wire diameters), are shown in figure 20. The zero degree value for pull strength of the

bonds with large bond deformation was approximately half that of the bonds with small bond

deformation. Bonds made on the third substrate had excessively large bond deformation but

yielded results essentially the same as those observed for bonds with large bond deformation.

The results show that as a increases, the measured pull strength decreases. This appears to be a result of the twisting or tearing of the bond heel as the wire is pulled out

of the plane normal to the substrate plane. These results show that, although the value

of angle a has little effect on the measured pull strength of bonds with small to moderate

24
bond deformation, the effect is more pronounced for the bonds with greater bond deformation

Although it was not experimentally demonstrated, it is hypothesized that if the failure mod

were by peel significantly lower pull strengths would occur at large values of a. This rea soning follows that presented in the introductory material. For peel failure at a=0, the

force tending to lift the bond acts uniformly over a large area. For a ^ 0, the force

tends to be concentrated on one side of the bond and hence initiates lift-off at a lower

pull force. It should be possible to control the position of the pulling hook so that the

angle a is less than 10 deg by visual inspection, but more precise positioning may be re- quired for bonds with large deformation [14].

5.2.3 TWO-LEVEL BONDS

The experiments on the effects of the angle a on the pull strength were carried out fo

small deformation two-level bonds in which the first bond was made on the high pad. Bond

pairs were made on two different bonding machines designated as machine A and machine B. The measured pull strength, normalized to its value for a = 0, is presented as a function

for a for both cases in figure 21. As with the analysis of the single level data, only

those failures due to rupture at the first bond were included in the data. The curves in

figure 21 show roughly the same general trends in both cases except for differences in the

standard deviations. The results are in general agreement with the measurements of pull

strength made on single-level bond pairs of small deformation [5].

T

4 8 12 16 20

ANGLE OF PULL a (deg)

Figure 21. Measured pull strength as a function of angle of

pull, a, for two-level bond pairs. (The values are normalized to the mean pull strength at a = 0 deg. Solid points

are for bond pairs made on bonding machine A. Open points are for bond pairs made on machine B. Error bars represent

one sample standard deviation above and below the mean.)

25
6. SENSITIVITY CALCULATION

The sensitivity of the measured pull strength of wire bonds to variations in experimental conditions was analyzed for the case, appropriate to unannealed aluminum ultrasonic

bonds, in which measured pull strengths are in agreement with those predicted by resolutionof-forces. The normalized variation in pull strengths of the bond to the terminal, AF(t),

and of the bond to the pad on the die, AF(d) , were studied as a function of the position of

the pulling hook, e; the loop height above the terminal post, h; the height of the terminal

post above the die, H; the bond-to-bond spacing, d; and the pull angle with respect to the

normal to the substrate, <j>

.

To illustrate the application of this analysis, computations were made for a transistor

in a TO-18 can. The TO-18 configuration was used as bond geometry variables were given in manufacturer's specifications. Each of the five quantities was varied about its nominal

value while the other four quantities were held constand, and AF(t) and AF(d) were computed.

The results are summarized in table 2; the quantities are listed in order of decreasing

effect on the measured pull strength. Except for variation of <j> , the variation is expressed

in percent of the nominal value.

It can be seen that, for the device described, the pull angle <J> has the greatest effect

on AF(t) and AF(d) . Of the five parameters, the pull test operator controls only (j) and e; the reproducibility of h and d is dependent on the bonding machine, and that of H is dependent on the header lot and die thickness. In making wire bonds in this laboratory, h and d can be reproduced to ±3.0 and ±3.8 percent (one standard deviation), respectively. Typical

manufacturer's specifications for TO can-type headers suggest that variations in post height

of ±30 percent may be expected. An estimate of the overall variation to be expected in determining pull strength was obtained by calculating values of AF(t) and AF(d) as a function

of <fi for two cases for the sample device. In one case each of the other four quantities as- sumed its smallest value, in the other each assumed its largest value. The results for

these simulations of the application of the pull test in an industrial environment, shown

in figure 22 indicate that, for the heel break (tensile) failure mode, variations in c|> of

10 deg can produce differences in pull strength from the zero degree case of as much as 30

percent [15]

.

26
Table 2 — Calculated Dependence of AF(t) and AF(d) on Pull Test

Conditions for a TO-18 Transistor

Quantity Change AF(tJ AF(d) Change AF(t J AF(d)

+10 deg -11% +14% +20 deg -19% +37%

-10 deg +20% -8% -20 deg +50 -13

d +10% -7% -6 +20% -13 -11 -10% +8% +6 -20% +16 +13

h +1 0% +5% +5 +20% +11 +10

-10% -5 -5 -20% -1 2 -1 0

e +1 0% +4 -3 +20% +9 -5 -10% -3 +4 -20% -6 +9

H +1 0% +2 +0.5 +20% +3 +1.0

-10% -2 -0.5 -20% -3 -1.0

AF(%)

80

20 30 40 50

I I

I

I II </>(deg)

Figure 22. Normalized variation in calculated pull strength

for the terminal bond, AF(t), and die bond, Af(d) as a function of pull angle cf>.

(For case L: d = 38.5 mils (0.98 mm);

h = "11.6 mils (0.29 mm);

e = 0.375; and

H = 3.5 mils (0.09 mm).)

(For case H:

d = 41 .5 mils (1 .05 mm);

h = 12.4 mils (0.31 mm);

e = 0.625; and

H = 6.5 mils (0.17 mm).)

27
7 . SUMMARY

It has been shown for 0.001 -in. (25-ym) diameter ultrasonically bonded aluminum (1%

silicon) wire that the variations in measured pull strength as functions of bond loop height

and position of the pulling hook are in agreement with those predicted by resolution-offorces for single- and two-level unannealed wire bond pairs provided that the ratio of the

loop height, h, to bond-to-bond spacing, d, is less than about 1/3. For annealed bonds,

agreement was not found. This was attributed to the fact that the stretching of the wire

loop during pulling always resulted in h/d values greater than 1/3. It should be noted that

these conclusions are drawn from those experiments where heel breakage (tensile failure) is

the exclusive failure mode.

The decrease in measured pull strength with increasing pull angle a in both singleand two-level bond pairs, more pronounced on bonds with large deformation, appears to be a result of the twisting or tearing of the bond heel as the wire is pulled out of the plane

normal to the substrate plane. In the series of measurements on the effects on pull strength

of varying the pull angle 6, for both single- and two-level unannealed wire bonds, a large

decrease in pull strength was observed as 3 went from -45 to 0 deg; as 3 went from 0 to +45

deg, the decrease was much less marked. This is consistent with the facts that the second

bond is stronger than the first bond and therefore the measured pull strength is lower when

the larger component of force is applied to the first bond, and higher when applied to the

second bond.

Examples of the practical applications of the work reported here are to be found in the

sensitivity calculation; the bond pull specifications for large-diameter wire; and a nondestructive bond pull test.

28
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3. Schafft, H.A. in Methods of Measurement for Semiconductor Materials, Process Control,

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also Ref 1, pp. 57-73)

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,

Process Control, and Devices, Bullis, W.M., Ed., NBS Technical Note 717 (April 1972),

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pp. 29-30.

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pp. 26-27.

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Materials, Process Control, and Devices, Bullis, W.M. , Ed., NBS Technical Note 754

(March 1973), pp. 21-23.

12. Leedy, K.O. and Ricks, D.R. in Methods of Measurement for Semiconductor Materials,

Process Control and Devices, Bullis, W.M., Ed., NBS Technical Note 560 (Nov. 1970),

pp. 30-31.

13. Leedy, K.O. and Main, C.A. in Methods of Measurement for Semiconductor Materials,

Process Control, and Devices, Bullis, W.M. , Ed., NBS Technical Note 592 (Aug. 1971),

p. 35.

14. Leedy, K.O., Sher, A.H., and Main, C.A. in Methods of Measurement for Semiconductor

Materials, Process Control, and Devices, Bullis, W.M. , Ed., NBS Technical Note 743

(Dec. 1972), pp. 27-28.

29
'5. Sher, A.H., in Methods of Measurement for Semiconductor Materials, Process Control,

and Devices, Bullis, W.M., Ed., NBS Technical Note 806 (Nov. 1973), pp. 28-30.

16. Harman, G.G. in Methods of Measurement for Semiconductor Materials, Process Control,

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17. Harman, G.G., A Metallurgical Basis for the Non-Destructive Wire-Bond Pull Test, 12th

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Proa. 1969 Annual Symposium on Reliability, Chicago, Illinois, January 1969, pp. 73-85.

19. Slemmons, J.W., The Microworld of Joining Technology, Proc. American Welding Society

50th Annual Meeting, Philadelphia, Pennsylvania, April 1969, pp. 22-27 , 35, 56, 72.

20. Polcari, S.M., and Bowe, J.J., Evaluation of Non-Destructive Tensile Testing, D0T-TSCNASA-71-10 (1971). (Available from National Technical Information Service, Springfield,

Virginia 22151, Accession No. N71-37516.)

21. Bertin, A. P., Development of Microcircuit Bond-Pull Screening Techniques, Rome Air

Development Center - Technical Report-73-123 (April 1973).

22. Harman, G.G., Metallurgical Failure Modes of Wire Bonds, 12th Annual Proceedings

,

Reliability Physics 1974, Las Vegas, Nevada, April 2-4, IEEE Catalog No. 74CH0839-1

PHY., pp. 131-141.

30
Appendix A Resolution -of-Forces Calculation

This appendix is intended to provide some of the details necessary to obtain eqs (1)

through (8) of Section 2 of this report. Referring to figure 1, the vector diagram of

figure Al may be constructed. Considering the applied pulling force at an infinitesimal

increment in force before rupture takes place, balancing of the forces along the x direction

and the y direction requires that

F ^ cos 6 = F sin d> + F , cos 6,, (Al) wt t wd d

and

F cos <)> Fwt

sin 6 t + Fwd

sin 6^ (A2)

respectively.

Solving eq (A2) forF - one obtains

wt

F cos d> - F , sin 6 . Fwt ~ ~ ^

sin 9

t

Substituting eq (A3) into eq (Al) and solving for F one obtains eq (1) wd

F . wd

(cos <|> ctn 9 - sin <f> \

~

I sin 6 d ctn 6 t + cos 6 d /

cos (9 + <{>) = F

£ sin (6 + 9 )

(1)

Substituting eq (A4) into eq (A2) and solving for F , one obtains eq (2): wt

p r cos (9 d ' *2 • (2)

wt sin (e

t + * d

)

31
To obtain eqs (4) and (5) one must make use of the following relations:

sin e„ = ^ •

cos 9, =

a

[(h+H) 2 + [(l-e)d] :

(l-e)d

[(h+H) 2 + [(l-e)d] 2

]

! (A4)

sin 6, [h2 + (ed) 2 )"

cos 9 ed

h

2 + ' ?Ah (ed) 2

]

c

1 vd Svtyj i P \ 1 ^"wdy

' wd\ | Fwtx

Fwdx

Figure Al . Vector diagram for resol ution-of- forces operative

in bonded system during application of pulling force. (The

following relations are used in obtaining Eqs. (Al ) and (A2).)

F

= X

F sin (f> F = F y

cos $

^wtx

= F . cos wt

9 t

F wtyF . sin

wt

9 t

^wdx

= F . cos

wd

e d

F . = wdy

F sin

wd

9 d

32
Appendix B Programs Used for Resolution-of-Forces Calculation

This appendix contains several programs which may be used to calculate F and F , wt wd

from the resolution-of-f orces calculation (See eqs (4) and (5)). These programs are in

HP-65 language, Basic, and Fortran. With the HP-65 program, the key entries of table Bl are entered into the calculator

and put on magnetic cards according to the instructions given in the owner's manual. The

program is then permanently entered on the magnetic cards.

To perform the calculation the following instructions are to be used. Enter the program. Clear the memory registers if they are full. Then, enter each of

the six variables in the memory indicated below. (There is no need to clear the stack

after each entry.)

Key Entry Stored Variables

STO 1 h

STO 2 H

STO 3 d STO 4 e STO 5 F STO 6 $ If any variable is zero, simply ignore that particular memory. Then press either A or B to obtain the desired result (either F or F ,). wt wd

Any or all of the variables may be changed by simply entering the new values in the

appropriate memories.

In the case of the angle <j>, which is normally expressed in degrees, this may be entered as either positive or negative depending upon the actual physical situation.

The values of h, H, and d may be entered in either English or metric units so long as they are consistent.

The value of F may be any set of units and need not be consistent with the units of h,

t

H, and d. The forces F and F , will, of course, have the units of F. wt wd

The remaining programs which may be used to calculate F and F are given in Basic

and Fortran. The programs have all the necessary information to allow for rapid computation. It should be noted that step 990 of the Basic program and the corresponding step of

the Fortran V program convert the pull angle <j> from degrees to radians. This is required

by the fact that both Basic and Fortran read angles in radians. However, the readout of

the angle is given in degrees. If a particular machine reads angles in degrees, simply set tt

A2=A1.

fThis program was written by G. G. Harman. The program was written for the HP-65 since it was the only programmable pocket calculator available at the time. Other programmable

pocket calculation may also be used for the computation,

ttThe Basic program, written with the assistance of W. J. Keery, is an adaptation of one used

by A. H. Sher to perform the sensitivity calculation (ref 15). The Fortran program was written with the assistance of T. F. Leedy.

33
Table Bl HP-65 Program for Resol ution-of-Forces Calculation

SWITCH TO W'PRGM PRESS [Tj | PRGM | TO CLEAR MEMORY. KEY ENTRY CODE

SHOWN COMMENTS KEY ENTRY

CODE

SHOWN COMMENTS REGISTERS

LBL 23 T 81 Ri h A 11 RCL 5 34 05

i f 31 X 71

STK 42 RTN 24 R, H RCL 1 34 01 LBL 23

RCL 2 34 02 B 12 1 1 h-± 1 61 f 31

RCL 3 34 03 STK 42

81 RCL 6 34 06

RCL 6 34 06 f 31 R4 e f 31 COS 05

SIN 04 RCL 4 34 04

X 71 X 71 R* F RCL 6 34 06 RCL 6 34 06 ..- - — —

f 31 f 31

COS 05 SIN 04 R6 * 1 01 RCL 1 34 01

1 t 41 RCL 3 34 03

RCL 4 34 04 81 R 7 used for] - 51 X 71 interX 71 - 51 n—

i

mediate

+ 61 RCL 2 34 02 R8 storage

STO 7 33 07 RCL 1 34 01

CL x 44 T 81

RCL 4 34 04 1 01 R9

RCL 3 34 03 + 61

X 71 X 71

RCL 1 34 01 STO 7 33 07 LABELS

81 CL x 44 A X

f

1 32 RCL 3 34 03 R X Jx 09 1 01 c

1 01 RCL 4 34 04 D

+ 61 — 51 E

f 31 X 71 O

/x 09 RCL 1 34 01 1 RCL 7 34 07 RCL 2 34 02 ? X 71 + 61

STO 7 33 07 81 4

CL x 44 f ~ 1 32 5

LBL 23 /x" 09 6

8 08 step 40 to 54 1 01 7

RCL 2 34 02 is a subroutine + 61 a

x RCL 4 34 04 used in both f 31 9

X 71 Fwt + Fwd r—° /x 09

RCL 1 34 01 RCL 7 34 07 FLAGS

1 81 X 71

1 01 STO 7 33 07

+ 61 CL x 44 2 RCL 7 34 07 g+0 22

g xf»y 35 07 8 08

TO RECORD PROGRAM INSERT MAGNETIC CARD WITH SWITCH SET AT W/PRGM.

34
Table B2 Basic Program for Resolution-of*-Forces Calculation

10 PRINT "DO YOU WANT ALL THE INTRODUCTORY COMMENTS? <YES=1. NO=2>"

20 INPUT N 30 ON N GO TO 40.380

40 PRINT

50 PRINT "******* RESOLUTION OF FORCES CALCULATION *******"

60 PRINT "******** DESTRUCTIVE BOND PULL TEST *********

70 PRINT

80 PRINT "THIS PROGRAM CALCULATES THE FORCES ON THE WIRE AT THE"

90 PRINT "TERMINAL AND AT THE DIE FROM THE BOND GEOMETRY AND THE"

100 PRINT " PULL STRENGTH"

110 PRINT

120 PRINT "B IS THE HEIGHT, AT RUPTUREi OF THE WIRE SPAN (AT THE " 130 PRINT " POINT OF PULL) ABOVE THE TERMINAL"

140 PRINT

150 PRINT "H IS THE HEIGHT OF THE TERMINAL RELATIVE TO THE DIE SURFACE"

160 PRINT

170 PRINT "D IS THE BOND-TO-BOND SPACING"

180 PRINT

190 PRINT "E IS THE D IMENS IONLE SS FRACTIONAL HORIZONTAL DISTANCE BETWEEN"

200 PRINT " THE TERMINAL BOND AND THE POINT OF APPLICATION OF THE " 210 PRINT " FORCE"

220 PRINT

230 PRINT "F IS THE PULL STRENGTH"

24 0 PRINT

250 PRINT "A IS THE PULL ANGLE AND IS EXPRESSED IN UNITS OF DEGREES"

260 PRINT

270 PRINT "Ti IS THE FORCE ON THE WIRE AT THE TERMINAL. FWT"

280 PRINT

290 PRINT "T2 IS THE FORCE ON THE WIRE AT THE DIE. F WD"

300 PRINT

310 PRINT "THE VALUES OF B. H, AND O MAY BE EXPRESSED IN EITHER"

320 PRINT "ENGLISH OR METRIC SO LONG AS THEY ARE CONSISTENT"

330 PRINT

340 PRINT "THE VALUE OF F MAY BE IN ANY SET OF UNITS AND NEED NOT BE"

350 PRINT "CONSISTENT WITH THE UNITS OF B. H, AND D" 360 PRINT

370 PRINT "THE FORCES TI AND T2 WILL. OF COURSE, HAVE THE UNITS OF F"

380 PRINT

39 0 PRINT

400 PRINT "ENTER B, H. D. E. F.A"

410 INPUT B.H.D.E.F.A

420 PRINT

430 PRINT "SPECIFY PARAMETER TO BE VARIED"

440 INPUT Q$

450 8 1= B 460 Hl = H

47 0 Dl = D 480 El = E 490 Fl = F

50 0 Al = A 510 IF E1>1 .0 GO TO 1110

52 0 IF Al>45.0 GO TO 1130

530 IF AK-45. 0 GO TO 1150

54 0 IF Q$="B" GO TO 640

55 0 IF Q$="H" GO TO 680

560 IF Q$="D" GO TO 7 20

57 0 IF Q$="E" GO TO 760

35
58 0 IF Q$="F" GO TO 810

590 IF Q$="A" GO TO 850

600 PRINT

610 PRINT •*!!!! INVALID PARAMETER ENTERED !»!!"

62 0 PRINT

630 GO TO 430

640 FOR B1=0.1*B TO 2»0*B STEP 0.1*B

650 GO SUB 950

660 NEXT Bl

670 GO TO 1170

680 FOR H1=0.1*H TO 2.0*H STEP 0«1*H

690 GO SUB 950

700 NEXT HI

710 GO TO 1170

720 FOR D1=0.1*D TO 2.0*D STEP 0.1*D

730 GO SUB 950

740 NEXT Dl

750 GO TO 1 170

760 FOR E1=0.1*E TO 2.0*E STEP 0. 1*E

770 IF El>1.0 GO TO 1110

78 0 GO SUB 95 0 79 0 NEXT El

80 0 GO TO 117 0 810 FOR F1=0.5*F TO 5.0*F STEP 0.5

820 GO SUB 950

830 NEXT Fl

840 GO TO 1170

850 IF 1 .0*A, 900. 860, 860

860 FOR A1=0.0*A TO 45.0*A STEP 5.0

870 IF Al>45.0 GO TO 1130

880 GO SUB 950

890 NEXT Al 900 FOR A1=0.0*A TO 45.0*A STEP -5.0

910 IF AK-45.0 GO TO 1150

920 GO SUB 950

93 0 NEXT Al

940 GO TO 1 170

950 B2=l .0+El*Hl/Bl

960 R1=E1*D1/B1

970 R2=1.0+R1*R1

980 R3=SQR(R2)

990 A2=0 .0174532925*A1

1 00 0 R4=( 1.0-E1)*COS( A2) +SIN( A2)*( Bl+Hl >/Dl

1010 T1=F1*R3*R4/B2

1020 Sl=(l .0-E 1 ) *Dl/( Bl+Hl

)

1C30 S2=1.0+S1*S1

104 0 S3=SQR(S2)

1050 S4=1.0+H1/B1

1060 S5=E1*C0S(A2)-SIN(A2)*B1/D1

1070 T2=F1*S3*S4*S5/B2

1080 PRINT Bl ,H1 ,D1 .El .Fl . Al ,T1 ,T2

1090 PRINT

1100 RETURN

1110 PRINT "E HAS EXCEEDED UNITY—THIS IS NON-PHYSICAL"

1 120 GO TO 1170

1130 PRINT "ANGLE HAS EXCEEDED 45 DEGREES"

1 140 GO TO 1170

1150 PRINT "ANGLE HAS EXCEEDED -45 DEGREES"

36
1160 GO TO 1170

1 170 PRINT

1 180 PRINT

1190 PRINT "DO YOU WANT TO RERUN PROGRAM? ( YES= 1 , N0= 2 >" 120 0 INPUT M 1210 ON M GO TO 1220,1250

1220 PRINT "DO YOU WANT TO VARY ONLY ONE PARAMETER? ( YES=1 . NO=2 )

1230 INPUT N 1240 ON N GO TO 430,400

1250 PRINT

126 0 STOP

1270 END

37
Table B3 Fortran Program for Resolution-of -Forces Calculation

DIMENSION IN(6)

DATA ( IN( I ) . 1=1 .6) / 1HB . 1HH, 1HO, 1HE. 1HF, 1HA/

WHITE (6.10)

10 FORMAT (IX,'* ****** RESOLUTION OF FORCES CALCULATION * * * 2* * * *»,/,•* ******* DESTRUCTIVE BOND PULL TEST * * * * 3* * * *• ,/IXi'THIS PROGRAM CALCULATES THE FORCES ON THE WIRE AT TH

4E' , 1 X, 'TERM INAL AND THE DIE FROM THE BONO GEOMETRY AND THE'. IX,

5 'PULL STRENGTH. '/IX. • B IS THE HEIGHT. AT RUPTURE. OF THE WIRE SPA

6N (AT THE* , IX , POI NT OF PULL) ABOVE THE TERMINAL' ./IX.

7 "H IS THE HEIGHT OF THE TERMINAL RELATIVE TO THE DIE SURFACE*. /IX

8,'D IS THE BOND TO BOND SPACING*)

WRITE (6.20)

20 FORMAT (IX, 'E IS THE D I MENS I ONL ESS FRACTIONAL DISTANCE ',/lX.

2 'BETWEEN THE TERMINAL BOND AND THE POINT OF •)

WRITE (6.30)

30 FORMAT ( 1 X . • APPL I C AT I ON OF THE FORCE './ IX .• F IS THE PULL STRENGTH'

2. /IX. 'A IS THE PULL ANGLE AND IS EXPRESSED IN UNITS OF DEGREES'./

3 IX, 'Tl IS THE FORCE ON THE WIRE AT THE TERMINAL. FWT'./1X.

4 'T2 IS THE FORCE ON THE WIRE AT THE DIE, FWD'./1X.

5 'THE VALUES OF B. H, AND D MAY BE EXPRESSED IN EITHER'. /IX.

6 'ENGLISH OR METRIC SO LONG AS THEY ARE CON S I ST ANT .

• ./ 1 X

7 'THE FORCES Tl AND T2 WILL, OF COURSE, HAVE •)

WRITE (6,40)

40 FORMAT (IX, "UNITS OF F. THE VALUE OF F MAY BE IN './IX,

2 * ANY SET OF UNITS AND NEED NOT BE CONSISTANT WITH'. /IX.

3 'THE UNITS OF B, H, AND D'

)

50 WRITE (6,60)

60 FORMAT (IX, 'ENTER B, H, D • E, F, A')

READ (5.290) B.H.D.E.F.A

7C WRITE (6,300)

READ (5,310) INQ

B1 =B H1=H

D1=D

E1=E

F 1=F

A1 = A IF (El.GT.1.0) GO TO 160

IF (A1.GT.45.) GO TO 210

IF (A1.LT.-45.) GO TO 240

DO 80 J=l,6

80 IF ( INQ.EQ. IN( J) ) GO TO (90,110.130.150.180. 200). J GO TO 7 0 90 DO 100 J=l ,20

100 CALL PRINT ( B* J*0 . 1 , HI , D 1 , E 1 , F 1 , A 1

)

GO TO 26 0

3.10 DO 120 J = l,20

120 CALL PRINT ( Bl , H* J* 0 . 1 . D 1 , E 1 , F 1 , A 1 ) GO TO 26 0

1 30 DO 140 J = l .20

140 CALL PRINT ( B 1 . H 1 . D* J* 0 . 1 . E 1 .F 1 , A 1

)

GO TO 260

ISO DO 170 J=l,20

IF (E1*J*0.1.GT.1.0) GO TO 160

GO TO 170

160 WRITE (6,320)

GO TO 260

38
170 CALL PRINT ( B 1 * H 1 . D 1 1 E* J* 0 . 1 ,F 1 , A 1 ) GO TO 260

160 DO 190 J=l«20

190 CALL PRINT ( Bl , HI , D 1 , E 1 1 F*J *0 . 1 , A 1

)

GO TO 260

200 IF (A) 230,,

DO 220 J=0,9

IF ( Al* J*5. GT. 45. ) GO TO 210

GO TO 220

210 WHITE (6,330)

GO TO 260

220 CALL PRINT < Bl , H 1 , O 1 , E 1 , F 1 , A *J * 5

)

230 DO 250 J=0,9

IF ( Al*J*5.LT.-45. ) GO TO 240

GO TO 250

240 WRITE (6,340)

GO TO 260

250 CALL PRINT ( B 1 , H 1 , O 1 ,L 1 , F 1 , A 1 * J*5

)

GO TO 260

260 WRITE (6,350)

READ (5,360) M GO TO (270,280), M 270 WRITE (6,370)

READ (5,380) N GO TO (70,50), N 280 STOP

290 FORMAT ()

300 FORMAT ( IX , 3 OHSPEC IF Y PARAMETER TO BE VARIED)

310 FORMAT (Al)

320 FORMAT (1X.46H E HAS EXCEEDED UNITY — THIS IS NON-PHYSICAL.)

330 FORMAT ( 1X.30H ANGLE HAS EXCEEDED 45 DEGREES)

340 FORMAT (1X.30HANGLE HAS EXCEEDED -45 DEGREES)

350 FORMAT (1X.43HD0 YOU WISH TO RERUN PROGRAM? (YES=1, NO=2))

360 FORMAT (II)

370 FORMAT (1X.61HD0 YOU WISH TO VARY ONLY ONE PARAMETER? (YES = 1 2 , NO =2))

380 FCRMAT ( II ) Cc ****************************************************************

c SUBROUTINE PRINT ( Bl , HI , Dl , E 1, F 1 , A 1

)

C

c ***************** ****************************;((#  ** *************

c 82=1.0+E1*H1/B1

R1=E1*D1/B1

R2=l .0+Rl*Rl

R3=SQRT(R2)

A2=A1*0 .0174532925

R4=( 1.0-E1)*COS(A2)+SIN(A2)*(B1+H1 )/Dl

T1=F1*R3*R4/B2

Sl = ( 1 . 0-E1 )*Dl/( Bl +H1 ) S2=l. 0+Sl*Sl

S3=SQRT(S2)

S4=l . 0+H 1/B1

S5=E1*C0S( A2)-SIN( A2)*B1/D1

T2=F1*S3*S4*S5/B2

WRITE (6,10) Bl ,H1 ,D1 ,E 1 ,F1 ,A1 ,T1 ,T2

RETURN

10 FORMAT (8( 1X.8G9.3)

)

END

39
Appendix C

Bond Pull Specifications for Large Diameter Wire

As was indicated in the Introduction, the better understanding of the pull test has

led to further developments. One of these, a bond pull specification for large diameter

wire, is included in this appendix.

At the request of the Defense Electronic Supply Center a bond-pull specification for

the 5- to 20-mil (0.13- to 0.51-mm) diameter aluminum wire used in JAN-TXV power transistors was developed for proposed inclusion in appropriate slash sheets to MIL-S-19500,

General Specification for Semiconductor Devices. This was a difficult problem because of

the many wire sizes involved (at least 10) , the wide range of geometrical configurations of

the transistors, and the large variety of metallurgical properties of the available wire.

In order to establish the basis for a realistic specification that offered a meaningful level of quality control and was acceptable to both users and manufacturers, five semiconductor plants were visited to obtain pull test data and other information. Two other

device manufacturers contributed additional data including mean-range charts for bond pull

tests, and acceptance criteria were supplied by several users. Three major bonding wire

companies supplied information on tensile strength, elongation, and other relevant metallurgical data. Additional bond-pull data were obtained in-house on a variety of wire sizes

in a limited selection of device types.

The specification developed covers bonds made of aluminum wire, with or without alloying additives, ultrasonically bonded to the die and either ultrasonically bonded or electrical-discharge welded to the post. The test, which is primarily for use on uncapped devices

being assembled at the manufacturer's plant, is performed by placing a rounded metal hook

under the span of wire midway between the two bonds and pulling vertically to destruction

with the device clamped. In some cases soldered clips of copper or other material are permitted. These may also be tested by this procedure, but in this case the hook is placed as close as possible to the soldered joint on the die. A bond failure is designated as any

bond or wire which breaks at an applied stress less than that indicated in table CI. Procedures are included in the specification for both bond failures and die fracture.

It should be noted from figure CI that, when plotted on log-log paper, the limits for

large diameter wire in table join smoothly with the limits for small diameter aluminum wire. For completeness these small wire limits are also included in the table [16]

.

The results of this investigation have been incorporated in MIL-STD-19500 slash sheets

for transistors.

40
Table CI — Bond Pull Limits

Wire diameter, mils (mm) Minimum pull limit, gf (N)

i • u \ u . yjcj j c

If) 09)

1.0 \ U . Uoo )

•3O In

I.J ^ U . U JO J A \ u . uh- y

o [V.v/O) 1 c

f n i 9 \ (0. 1 £

j

5 (0.13) 30 (0.29)

8 (0.20) 75 (0.74)

10 (0.25) 120 (1.18)

15 (0.38) 220 (2.16)

20 (0.51) 300 (2.94)

solder cl ips 300 (2.94)

100 F

MINIMUM BOND PULL LIMITS (grams-force)

Figure Cl . Minimum destructive bond-pull breaking-force vs wire diameter for aluminum wire bonds in semiconductor devices.

41
Appendix D

Non-Destructive Wire Bond Pull Test

As the bond pull test is destructive to the bond, a non-destructive bond pull test has

been introduced and used by the electronics industry. However, the stress level for which

the test is truly non-destructive both in the short-term and long-term sense is purely a matter of trial and error for each individual case. The work done on and the understanding

resulting from the destructive bond pull test as described in this report have led to certain criteria of general applicability for the force level to be safely used in the application of the non-destructive bond pull test. The results of this investigation for aluminum and gold wire are presented in this appendix.

In view of the fact that non-destructive bond pull (NDP) tests are often used in highreliability production lines, the metallurgical and statistical rationales for the test developed, and maximum permissible NDP force limits applicable for a wide variety of wire

sizes, both hard drawn and annealed were determined [17]. Previous work [18-21] on the

subject has been restricted to small diameter wire having relatively low elongation under

tensile stress. The NDP test consists of pulling bonds at force levels below those necessary to cause breakage of the bond loop system. In order for the NDP test to be truly non-destructive

and hence avoid the introduction of metallurgical defects in the bond loop system, no part

of the bond loop system should be subjected to stresses greater than the metallurgical

elastic limits. For the NDP test to be statistically meaningful, it must be carried out in conjunction

with a destructive bond pull test. The results of both the destructive bond pull test and

the NDP test depend upon the same variables, including wire metallurgical properties, bonding technique, bond geometry, and bond deformation. Hence the mean and the standard deviation of the destructive bond pull test are significant factors in developing criteria for

the NDP test.

In relating the results of a destructive pull test to criteria for a non-destructive

pull test the stress-strain relationship of the wire is a significant factor. The effect

of the state of hardness on the stress-strain relation is exemplified in figure Dl . In this figure stress-strain curves are shown for the same aluminum wire in two states of

hardness. The annealed wire has a large elongation typical of large diameter aluminum wire

used bonding power devices. Hard-drawn or stress-relieved wire has a small elongation and

is typical of that of small-diameter aluminum, wire used in bonding integrated circuits.

Bonds made with small diameter aluminum wire are typically ultrasonically welded which

overworks the bond heel and causes it to be the weakest part of the bond-loop system.

Hence, the strength of the heel determines the maximum safe NDP force. The heel strength

depends upon the bond deformation which in turn depends upon the bonding schedule. Hence,

a safe NDP force may be determined only if a destructive bond pull test is performed on

bonds made under the same conditions. The results of a destructive bond pull test are assumed to follow a normal distribution for which x is the mean breaking force and s is the

42
Figure Dl . Stress-strain curves for annealed (A) and harddrawn or stress-relieved (B) aluminum wire. (This figure

was made up from individual stress-strain charts reversed

in direction for clarity of presentation. In order to display both curves on the same chart the stress axis was made relative, since the breaking load of the annealed

wire was approximately one-half that of the hard-drawn

wire. On both curves region 1 is the elastic region

where the stress is proportional to the strain, point 2 is the proportional or elastic limit, region 3 is the

region of inelastic or plastic deformation, and point 4 is the breaking load of the wire.)

Table Dl — Summary of NDP Force Recommendations

Type of Wire Relation Between x and s Recommended

Production Line Composition Elongation on the Bond Pull Test Maximum Safe

NDP Force

Normal Aluminum <3% 0.15x<ss0.25x 0.9(x - 3s)

High Rel Aluminum <3% s$0.15x 0.9(x - 4s)

All Aluminum ^5 to 20% s$0.25x (x - 3s)/2

Aluminum >20% ss0.25x (x - 3s)/3

All Gold Any 0.15x<s$0.25x 0.7 (x - 3s)

Gold Any ssO.l 5x 0.7(x - 4s)

43
sample standard deviation of the distribution. For normal assembly lines, where s can be

expected to lie in the range 0.15 x to 0.25 x the recommended maximum safe NDP force is

0.9 (x - 3s). For high-reliability lines, s may be equal to or less than 0.15 x. For this

case, the recommended maximum safe NDP force is 0.9(x - 4s). No NDP testing is recommended

for cases where s is greater than 0.25 x since this indicates that some aspect of the bonding procedure is out of control and either a low, meaningless NDP force would be used or

too many bonds would be stressed beyond their elastic limits.

The large diameter aluminum wire used in power devices typically has an elongation up

to 20 percent before breaking. Also, this wire is usually ultrasonically bonded with a grooved capillary-type tool which limits the maximum bond deformation and hence does not produce a weak bond heel. In this case, the stress-strain relation for the wire span determines the maximum safe NDP force. Due to the differences in the stress-strain relation

from that of small diameter wire, a different NDP force is necessary. For a wire elongation

up to 20 percent and an s of 0.25 x or less the recommended maximum safe NDP force is

(x - 3s)/2. For an elongation greater than 20 percent, the maximum NDP force must be re- duced to (x - 3s) /3 in order to avoid significant inelastic wire elongation during the

test. Gold wire bonds formed by thermocompression or ultrasonic techniques do not have weak,

overworked bond heels. Hence, the stress-strain relation for wire span determines the NDP

force. These relations for small diameter grain- stabilized gold wire used in both ultrasonic and thermocompression bonding show that this type of gold wire typically experiences

most of its elongation above 85 to 90 percent of its breaking load. However, some other

gold wires may begin to elongate as low as 70 percent of their breaking load. Considering

this and the high pull strength, even with large bond deformation, the recommended maximum

safe NDP force control limit to include all small diameter gold wire is 0.7(x - 3s) or 0.7(x -4s) for 0.15 x < s < 0.25 x or s <0.15 x, respectively.

Table Dl summarizes these recommendations. It must be noted that the NDP test ensures

the reliability of a bond at the time of the test but does not prevent the later failure of

the bond from intermetallic compound formation or post-production stresses [22]

.

44
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS Spec. Publ. 400-182. Gov't Accession

No.

3. Recipient's Accession No.

4. title and subtitle Semiconductor Measurement Technology:

The Destructive Bond Pull Test

5. Publication Date February 1976

6. Performing Organization Code

7. AUTHOR(S) ... rJ..„ John Albers, Editor 8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Taslc/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

NBS - Washington, D. C. 20234

USNSSPO - NAD Crane, Ind. 47522

DNA - Washington, D.C. 20305

13. Type of Report & Period Covered Final

14. Spo^s^orjjij Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 75-619425

16. ABSTRACT (A 200-word or less (actual summary ol most significant information. If document includes a significant bibliography or literature survey, mention it here.) This report summarizes the work done at NBS on the destructive bond pull test as applied

to small -diameter (approximately 1 mil or 25 urn) ultrasonically bonded; aluminum wire.

This work was performed during the period from 1969 to 1974. The report begins with a brief summary of the calculation of the resolution-of-forces operative in the bond

system during the application of the pulling force. Next, comparisons of the theoretical and experimental dependencies of the pull strength on the variables involved in

the resolution-of-forces calculation are given. Some of the variables which are not

directly involved in this calculation are then considered and their effects on the

measured pull strength are presented. The report ends with a sensitivity calculation

as to how well the variables must be controlled to maintain the variability of the pull

strength to within given limits. Bond pull specifications for large-diameter wire as

well as recommended force levels to be used in the application of the nondestructive

bond pull test, both of which have resulted from the pull test work, are considered in

the appendices.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Bond angle; bonding; bond pull test; bond-to-bond spacing;

large wire; loop height; microelectronics; non-destructive bond pull test; position of

hook; pull rate; pull strength; resolution-of-forces; semiconductor devices;

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